Haemophilus ducreyi is a strict human pathogen that causes chancroid, a genital ulcer disease (GUD) that facilitates the transmission of the human immunodeficiency virus (HIV-1). To study the biology of H. ducreyi, we developed a human challenge model that closely simulates natural infection. During infection, H. ducreyi is found in the hostile environment of an abscess and resists phagocytosis. We found that H. ducreyi had a broad transcriptional response to the host, suggesting that H. ducreyi senses and responds to the host environment. H. ducreyi contains only two systems known to respond to extracytoplasmic stress: the 2- component regulator CpxRA and the alternative sigma factor, RpoE. Several H. ducreyi homologues of CpxRA-specific effectors were upregulated, while many homologues of RpoE-specific effectors were downregulated in pustules, suggesting that that CpxRA and RpoE systems are linked in H. ducreyi and function in a coordinated fashion to respond to stresses mounted by the host. We found that CpxRA controlled the expression of several major virulence determinants of H. ducreyi and that uncontrolled activation of CpxRA impaired the ability of the organism to infect human volunteers. We hypothesize that H. ducreyi senses the host environment via CpxRA and RpoE, that both CpxRA and RpoE function during infection to combat stresses encountered in vivo and control the production of virulence determinants, that constitutively active expressers or deletion mutants in the CpxRA or RpoE systems will be attenuated for virulence and that known or novel virulence determinants will be differentially regulated by CpxRA and RpoE during infection. To test these hypotheses, our specific aims include: 1) evaluation of cpxR, rpoE, rseA and rseC deletion mutants and strains that express constitutively activated CpxR and RpoE for virulence in human volunteers; 2) determination of the mechanism(s) underlying the contributions of the CpxRA and RpoE systems to pathogenesis in models relevant to human infection; 3) identification of novel virulence determinants controlled by the CpxRA and RpoE pathways and evaluation of their role in infection in the in vivo (Aim1) and in vitro (Aim 2) models. Our proposal offers the unique opportunity to study the contributions of two interrelated stress response systems to the survival of a pathogen in humans, will lead to the identification of novel virulence determinants, and has already led to the development of a novel antimicrobial strategy. As approved by NIAID, the purpose of this revision is to obtain funds to support the human inoculation experiments related to Aims 1 and 3 of the funded application. PUBLIC HEALTH RELEVANCE: Haemophilus ducreyi is a bacterium that causes a genital ulcer disease that facilitates HIV transmission. H. ducreyi turns on the expression of many of its genes when it infects the human host. Our study is designed to examine how H. ducreyi uses stress response systems to adapt to the human host. H. ducreyi is only killed by three classes of antibiotics and is a high priority organism for the development of new therapies. This project should yield targets of vaccines or alternative treatments for H. ducreyi and other bacterial infections.